Research is proposed to address Objective 3 in Program Announcement PA04052. The introduction of a single WNV strain at a relatively well characterized time and place in North America has provided a unique opportunity to prospectively study the evolution of an arthropod-transmitted RNA virus as it adapts to a naive ecological niche. While WNV in the Americas remains a relatively homogeneous virus population, a single genotype that differs from the introduced genotype has arisen, become dominant throughout much of North America, and since 2002 has largely displaced previously existing genotypes in New York. Preliminary studies suggest that the dominance of this new genotype may be facilitated by interactions between the virus and the mosquito host. The range of biological mechanisms that may lead to the displacement of one arbovirus genotype with another, however, have not been systematically examined. Furthermore, the range of phenotypic expression that may accompany genetic variation in WNV and other arthropod-borne viruses is not well characterized and difficult to predict. Accordingly, we will test the hypothesis that dominance of one WNV genotype may result from selective advantages of this genotype in the mosquito and/or avian host. Specifically, proposed studies will focus on whether this dominance is achieved through differential potential for genetically defined WNV strains to (a) produce vertebrate viremia, (b) be transmitted orally by mosquitoes, and (c) be transmitted vertically from infected female mosquitoes to their offspring. Further, we propose experiments that will determine whether these phenotypic differences are associated with differences in relative viral fitness. Finally, we will determine the point(s) in the WNV transmission cycle that are likely to have been responsible for the observed displacement of one WNV genotype with another. Results from these studies will dramatically increase our understanding of the selective processes that shape natural arbovirus populations.